The origins of the Mengye potash deposit in the Lanping–Simao Basin, Yunnan Province, Western China

Ore Geology Reviews 69 (2015) 174–186

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The origins of the Mengye potash deposit in the Lanping–Simao Basin, Yunnan Province, Western China Minghui Li a,b,⁎, Maodu Yan a,c, Zhengrong Wang d, Xiaoming Liu a,c, Xiaomin Fang a,c, Jiao Li a,e a

Institute of Tibetan Plateau Research, Chinese Academy of Sciences (CAS), Beijing 100085, China Key Laboratory of Tibetan Environment Changes and Land Surface Processes, CAS, Beijing 100085, China Key Laboratory of Continental Collision and Plateau Uplift, CAS, Beijing 100085, China d Department of Earth and Atmospheric Sciences, City College of New York, CUNY, New York, NY 10031 e University of Chinese Academy of Sciences, Beijing 100049, China b c

a r t i c l e

i n f o

Article history: Received 9 September 2014 Received in revised form 1 February 2015 Accepted 3 February 2015 Available online 7 February 2015 Keywords: Potash deposit I/Cl Br/Cl Strontium isotope Boron isotope Sulfur isotope

a b s t r a c t The Lanping–Simao Basin (LSB) is a Mesozoic–Cenozoic continental margin rift basin in Western China. It formed during the opening and closing of the Tethys Ocean. This basin is also known as a “metal belt” as it hosts several metal deposits, besides, the Mengye potash deposit. However, the exact dates of the formation either in the Paleocene or the Cretaceous, and thus the origins of the marine, continental or mixed origins of the Mengye deposits, remain disputed. Based on the basin's evolution, materials of marine origin and/or remnant seawater should be present, but instead the salt layers of the Mengye potash deposit present typically continental lithological features. This study examines and reviews evaporative minerals, Br/Cl and I/Cl molar ratios, and isotopes of S, B, and Sr·I and I/Cl data for this area has not been previously reported. The basin's evaporative minerals are dominated by halite and sylvite. The amounts of anhydrite, chlorocalcite, langbeinite, glaserite, tachyhydrite and glauberite are small. All of these form in both marine and continental environments. The values of I and the I/ Cl molar ratios of halite and sylvite are from 0.07 to 0.27 ppm, and from 0.03 to 0.11 × 10−6, respectively, dependent on organic substances. Br and molar Br/Cl values are from 89.08 to 555.45 ppm and from 0.06 × 10−3 to 0.38 × 10−3, respectively. All of the Br/Cl molar ratios are lower than those of seawater, and most of them are b 0.1, suggesting continental or mixed origin. Previously published δ34S, δ11B and 87Sr/86Sr values for evaporative minerals indicate a continental origin for the Mengye potash deposit. However, materials of hydrothermal origin are widely distributed in the basin and may have played an active role for the formation of the potash deposit. Thus the Mengye potash deposit could be of continental origin, with a remnant seawater trace. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The occurrence, distribution and origin of potash evaporite deposits have been the subject of many previous studies owing to their economic, sedimentary, geological and climatic significance (e.g. Warren, 1989, 2006, 2010; Utha-Aroon, 1993; Qian et al., 1994; El Tabakh et al., 1998, 1999; Rahimpour-Bonab and Kalantarzadeh, 2005; Cendón et al., 2008; Talbot et al., 2009a, 2009b; Tan et al., 2010; Tong et al., 2013; Zhang et al., 2013). Potash deposits typically originate from brines of marine or/and land origin(s), along with varying inputs from deeplycirculating meteoric, basinal and hydrothermal fluids (e.g. Warren, 2006). Almost all large potash deposits are associated with marine fluids (such as the deposits in Thailand, the United States, Germany, Russia, France and Brazil: Qian et al., 1994), whereas some small potash deposits are originated from continentally-sourced fluids (such as ⁎ Corresponding author at: Institute of Tibetan Plateau Research, Chinese Academy of Sciences (CAS), Beijing 100085, China. E-mail address: [email protected] (M. Li).

http://dx.doi.org/10.1016/j.oregeorev.2015.02.003 0169-1368/© 2015 Elsevier B.V. All rights reserved.

deposits in the Qaidam Basin (QB), western Tibet, China: Chen and Bowler, 1986; Lowenstein et al., 1989). Previous studies have also suggested that potash deposits could form in an (semi-) isolated tectonic basin under arid climate conditions with a large fluid influx (e.g. Qian et al., 1994). Arid climate creates a condition whereby evaporation rate is faster than inflow rate, so that evaporites precipitate out of concentrated brines in restricted basins. Today, exploited evaporite deposits are found mostly in the arid and semiarid deserts of the world. Tectonics and climate, not eustasy, are thus the prime controlling factors of most evaporite deposits. The Lanping–Simao Basin (LSB) in southwestern China is located at the junction of the Eurasian and Indian Plates on the eastern Tibetan Plateau (TP), with a complex tectonic, geologic, and evolution. As a Mesozoic–Cenozoic sedimentary basin, it is circumscribed by two deepseated faults: the Lancangjiang Fault to the west and the Jinshajiang– Ailaoshan Fault to the east (Fig. 1). The basin is also known as a “metal belt” hosting several other well-known metal deposits besides the Mengye potash deposit (e.g. the Jinman Cu and Jinding Pb–Zn deposits). It is currently filled with Late Triassic to Eocene sediments dominated by

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175

Fig. 1. Sketch geological and tectonic map showing samples of localities. After Google earth and Xue et al., 2002).

terrestrial siliciclastic rocks, with several intervals dominated by evaporites. Salt-bearing sediments occur in the strata of the Late Triassic, Middle Jurassic and Paleocene (Qu et al., 1998). The small-scale salt beds of the Late Triassic and Middle Jurassic are composed of halite, gypsum and brines and occur sporadically throughout the basin. The main potash-forming stage has been described as the Mengye Formation. Because the basin is adjacent to both the large Maha Sarakham potash deposits found in the Khorat Basin (KB) in Thailand and the allied Tagong deposit located in the Sakon Nakhon Basin (SNB) in Laos (Fig. 1), it has been proposed that, due to the occurrence of similar salt minerals within the same tectonic belt in both these and the Lanping–Simao areas, the Mengye potash deposit in the LSB should also be large and important. The potash would have formed in brines from similar sources (e.g. Qu et al., 1998; Li and Hu, 2001; Li et al., 2003), due to the occurrence of similar salt minerals in both these areas within the same tectonic belt. Surprisingly, in ~ 50 years of exploration, no large potash deposit has been discovered in the area, and no consensus has therefore been reached regarding the fluid origin of the LSB, or indeed that of the potash deposits in the SNB and KB (Utha-Aroon, 1993; El Tabakh et al., 1998, 1999; Tan et al., 2010). In this study we examined 27 salt samples from the LSB and SNB in Laos to establish their mineralogy, total carbon content, and Br/Cl and I/Cl molar ratios (Fig. 2). This is the first time that the iodine (I) concentration and I/Cl ratios in the halite and sylvite are reported. These data, combined with the published geochemical data of evaporite minerals (including Br/Cl, 87Sr/86Sr, δ11B and δ34S), may help to constrain the origins of the Mengye potash deposit in the LSB, China. 2. Geological background The LSB became a Mesozoic–Cenozoic continental margin rift basin during the collision between Eurasian plates (Liao and Chen, 2005). It has transitioned from an oceanic to an oceanic–continental, and then continental basin. According to previous studies (Mou et al., 1999; Fu, 2005; Liao and Chen, 2005), the basin was formed during the Middle Triassic accompanying the opening of the Mid-Tethys Ocean. During the Middle and Late Triassic, fault systems developed, allowing a wide depression, with semi-deep marine deposition, from the margin to the center of the basin. Sediments were dominated by marine sandy and

muddy flysch in the early Triassic, alternating sea-land felsic volcanic rocks in the Middle Triassic, continental clastic rocks, mafic and intermediate volcanic rocks, and felsic and intermediate rocks in the Late Triassic (Qu et al., 1998). The central LSB sat in a shallow marine environment with stable carbonate deposits, and transgressions became common throughout the basin. The closing of the Mid-Tethys Oceanic arm caused crust deformation during the Lower Jurassic, and seawater withdrew until the opening of the Neo-Tethys Ocean during the Middle Jurassic, which in turn resulted in further subsidence of the basin. At the end of the Jurassic, the Neo-Tethys closed and the crust began to uplift. The LSB became an alternating marine-continental basin. Sediments were dominated by fine red clastic rocks with gypsum and brine in the early Jurassic, red and gray–green clastic rocks with gypsum and mudstone conglomerates in the Middle Jurassic, and red clastic rocks in the Late Jurassic (Qu et al., 1998). Since the Cretaceous, the LSB has been relatively stable within the Eurasian continental plate (Cung and Geissman, 2013). According to Qu et al. (1998), in the Early Cretaceous the basin changed from gulf to coastal plain and finally to alluvial plain, dominated by sandstone sediments. In the mid-Cretaceous both the LSB and the KB (including the SNB) may have been desert environments (Fig. 3b), suggesting that potash deposits were possibly formed either at that time, or earlier. From the Late Cretaceous to the Eocene, the LSB exhibits a transition from a back-arc foreland basin to a strike-slip pull-apart basin (Mou et al., 1999). In the Early Eocene, the area experienced several significant rotational deformations, especially after the collision of the Indian and Asian plates (Sato et al., 2007; Tong et al., 2013). Based on paleomagnetic reconstructions, the penetration by the Indian Plate was accommodated by a clockwise rotational motion and southward displacement (Sato et al., 2007). The central part of the Indochina Block (the Khorat Plateau) is relatively rigid with respect to the Simao Terrane (which includes the LSB), and has been experiencing clockwise rotation of ~15° up to the present day (Yang and Besse, 1993; Yang et al., 1995). The central part of the Simao Terrane (including the LSB) has been subjected to a clockwise rotation of ~50° and has undergone an 800 km southward displacement from the Late Eocene to the present. The northern sector of the Simao Terrane (including the LSB) has experienced a clockwise rotational motion of ~30° and a southward displacement of more than 1000 km (Funahara et al., 1992; Sato et al., 1999, 2001; Yang et al.,

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Fig. 2. Distributions of samples (a) and hand specimens from China (b–d) and Laos (f–g). (b) (No. MY640w-1), gray halite (molar Br/Cl of 0.08) intersected by red sylvite (molar Br/Cl 0.38); (c) (No.670-0), alternating gray and white halite; (d) (No. MY-syl) greenish sylvite and halite; (e) (No. GM-9), red sylvite; (f) (No. GM-10) gray halite; and (g) (No. GM-2-8) white halite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Locations of Lanping–Simao Basin from paleo-magmatic results (a) in the Cretaceous (after Huang and Opdyke, 1993; Li et al., 1999, 2004a; Sato et al., 1999, 2001, 2007; Yang et al., 2001; Morley, 2012; Cung and Geissman, 2013; Tong et al., 2013); (b) in the Mid-Cretaceous (after Hasegawa et al., 2010); and (c) in the Paleocene and Eocene (after Chen et al., 1995; Li et al., 1999; Sato et al., 2001; Cung and Geissman, 2013). Paleomagnetic data suggest that the Lanping–Simao Basin moved southward approaching Khorat Basin.

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Table 1 Evaporite minerals and their parent brines (after Hardie, 1984). Brine composition and its source

Major saline minerals

Key minerals

I. Na–K–CO3–Cl–SO4 Non-marine waters (particularly meteoric)

Alkaline earth carbonates, gaylussite, pirssonite, mirabilite, thenardite, trona, nahcolite, natron, thermonatrite, shortite, burkeite, aphthitalite, halite, sylvite, hanksite Alkaline earth carbonates, gypsum, anhydrite, mirabilite, thenardite, glauberite, polyhalite, epsomite, hexahydrite, kieserite, bloedite, and other Na–Mg sulfates, leonite and other K–Mg sulfates, kainite, carnallite, halite, sylvite, bischofite Alkaline earth carbonates, gypsum, tachyhydrite, antarcticite, anhydrite, sylvite, carnallite, bischofite

Na2CO3 minerals

II. a) Na–K–Mg–SO4–Cl seawater and non-marine waters (meteoric/volcanogenic) or mixed marine and non-marine waters II. b) Na–K–Mg–Ca–Cl non-marine waters (particularly hydrothermal)

2001). The location of the LSB has thus changed from 25.7°N in the Cretaceous to 18.6°N in the Paleocene and Eocene (Fig. 3a, b and c). This suggests that the LSB has moved gradually southward toward the KB (which includes the SNB). According to Qu et al. (1998), the lacustrine sediments of the Mengye Formation from bottom to top are: (1) red muddy conglomerate, fine sandstone and evaporites with brownish muddy fine sandstone; (2) purplish fine sandstone and mudstone; and (3) purplish conglomerate and mid-fine sandstone. The two salt-bearing strata are composed of: (a) red silty sand with a muddy interlayer; and (b) brownish red and motley muddy conglomerate mixed with thin mud and muddy limestone. The thickness of the sylvite layers is ~ 200 m and ~ 400 m. The age at which the Mengye potash deposit developed remains disputed. Most previous studies have argued that it developed during the Paleocene (e.g., Qu et al., 1998; Xu, 2008), but, based on pollen analysis, it has now been placed between the Aptian and Albian periods of the late Early Cretaceous (Yuan et al., 2013). Regardless of this, the Mengye potash deposit may provide constraints on regional climate conditions during the evolution of the basin and the compositions of fluids from which evaporite minerals were precipitated. As such, it may represent a record of the area's marine origins or remnant seawater. Modern evaporite deposits are mostly found in arid and semiarid deserts of the world, and the majority were formed in belts (between latitudes 15° and 45° north and south of the equator) (Qian et al., 1994; Warren, 1989, 2010). Based on sedimentological and palynological

MgSO4 minerals or Na2SO4 minerals

KCl ± CaCl2 minerals in the absence of Na2SO4 and MgSO4 minerals

analyses, the deposition age of the Mengye potash deposit is either in the Paleocene or Cretaceous, respectively (Qu et al., 1998; Yuan et al., 2013). Paleomagnetic studies show that the LSB was within the arid belt of 15–45°N during the Cretaceous and Paleocene (Fig. 3), and was dominated by a subtropical high-pressure system, indicating favorable conditions for the formation of potash deposits (Hasegawa et al., 2010). In the mid-Cretaceous, both the LSB and the KB (including the SNB) were within a subtropical high-pressure belt (Hasegawa et al., 2010). However, the scale of the Mengye potash deposit in the LSB is much smaller than that of the KB and SNB (Qu et al., 1998). This surprising contrast can be explained by the following possibilities. First, part of the LSB lay outside the arid belt during the Paleocene (Fig. 3c). Second, the age of the Mengye potash deposit in the LSB remains uncertain. Finally, because salt is mechanically weak and flows like a fluid under pressure, the section of the Mengye potash deposit that formed during the Cretaceous has been either dissolved or moved by tectonic activities in the area. The rheology and incompressibility of salt makes it inherently unstable under a wide range of geological conditions (Hudec and Jackson, 2007). In the LSB, all of the salt bodies are lensoid in shape, with an echelon arrangement along the strike (Qu et al., 1998). This may have resulted from the movements and deformations of salt bodies during tectonic activities. Because the basin evolved from marine environments and was finally filled with terrigenous sediments, the Mengye potash deposit should exhibit evidence of continental origins with remnant marine brines.

Fig. 4. Secular variation in potash evaporite and major ionic composition of seawater over the past 550 Ma. After Hardie (1996).

178

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Table 2 Evaporite minerals from potash deposit in China and in Laos. Location Mengye Formation in Lanping–Simao Basin, China

MengYe mine

Other minor/trace minerals by Qu (1997) and Han et al. (2011) Upper member in Tagong Formation, Laos

Mengye

Middle member in Tagong Formation, Laos

Core Zk204 Core Zk40308

Core Zk40308

Bottom of the middle member

Lower member in Tagong Formaiton, Laos

Core Zk204 Core Zk40308

In Dongtai mine

Number

Sample type

Main minerals

Minor/trace minerals

MY-670-0

Gray salt

Halite (NaCl)

Sylvite, anhydrite (CaSO4), glauberite (CaSO4·Na2SO4), trace chlorocalcite (KCaCl3) Anhydrite Anhydrite, glauberite and trace dolomite (CaMg(CO3)2) Glauberite Chlorocalcite, langbeinite(2MgSO4·K2SO4) Sylvite –

MY640W-1R Vein salt in the gray salt MY640W-1S Gray salt with red vein salt

Halite, sylvite (KCl) hAlite

MY670-W5 MY670W-1 MY670W-7 MY-670-1

Redish salt Gray salt Red salt vein Salt

MY-670

Brownish Salt

Halite Halite Halite, glauberite Halite, sylvite, anhydrite Halite

Glauberite

W-5 MY-syl Blue green Salt Sylvite, halite Anhydrite and quartz Carnallite(KMgCl3 6H2O), gypsum(CaSO4·2H2O), magnesite (MgCO3), rinneite (K2NaFeCl6), CaSO4·1/2H2O, kalistrontite (K2SO4·SrSO4), thenardite(Na2SO4), and koenenite(Mg5[Na6Al2(Cl,Br)6·(OH)11·nH2O),α-boracite (Mg3B7O13Cl), parahilgardite (Ca2B5O9Cl·H2O), celestite (SrSO4) GM-6 Pink earthy salt Anhydrite Trace halite GM-7 White salt Halite, sylvite Trace anhydrite GM-8 Red salt Halite and anhydrite Trace chlorocalcite GM-3 Gray and white salt Anhydrite Halite and glauberite GM-9 Red striation salt Dylvite, carnallite GM-10 Gray salt Halite Anhydrite, glauberite and trace chlorocalcite GM-2-2 White and transparent salt Halite Anhydrite, glauberite and chlorocalcite GM-2-3 White and transparent salt Halite and anhydrite GM-2-4 Red and white salt Halite, sylvite Anhydrite, glauberite, trace chlorocalcite GM-0-2 White and transparent salt Halite Anhydrite, glauberite and chlorocalcite GM-0-3 White and transparent salt Halite, sylvite Anhydrite, glauberite and chlorocalcite GM-4 Salt Halite Anhydrite GM-11 White with red point salt Halite Anhydrite and glauberite GM-12 White transparent with Halite Trace glauberite pink point salt GM-2-5 Brownish salt with clay Halite Sylvite, anhydrite, glauberite and trace chlorocalcite and dolomite GM-2-6 Gray salt Halite, sylvite Anhydrite and trace chlorocalcite GM-2-7 White, non-transparent Halite, sylvite Anhydrite salt GM-2-8 White and transparent salt Halite Anhydrite and trace chlorocalcite

How such marine evidences can be identified is an important question. Mineral, geochemistry and isotope proxies will therefore be discussed in the following sections. 3. Mineralogy of evaporites The chemical compositions of major ions in solutions dictate the mineralogy of precipitates. Because evaporite minerals precipitate directly from water, they can indicate the hydro-geochemical origins and conditions extant at the time of their precipitation. When all major ions precipitate from modern seawater, the precipitates can contain 88.64%, 10.8% and 0.34% of chloride, sulfate and carbonate, respectively, whereas river water has a much higher abundance of carbonates, but a similar proportion of sulfate(with precipitates containing 5.2%, 9.9% and 60.1% of chloride, sulfate and carbonate, respectively; e.g. Liu, 2002). Although the chemical compositions of seawater have varied dramatically over geologic time (Hardie, 1996; Horita et al., 2002; Siemann, 2003; Timofeeff et al., 2006), saline Na-carbonates never form in marine evaporites among the reported salt minerals (N 200) (Qu et al., 1979; Warren, 1989). This could be a constraint on either continental or marine environments. Ionic constituents of non-marine water are more diverse and less predictable than in seawater. For example, Na2CO3-minerals, trona, gaylussite, pirssonite, burkeite, northupite and hanksite occur mostly in saline continental settings (Table 1). Tachyhydrite (CaMg2Cl6·12H2O) and antarcticite (CaCl2·6H2O) are present mostly in non-marine settings (particularly under hydrothermal regimes). Halite, gypsum, sylvite and carnallite are dominant in most evaporite and potash deposits (Warren,

1989; Qian et al., 1994). Although chlorides are minimal in continental waters, they can form both in marine and non-marine settings (Table 1), and gathered in continental potash deposits such as Qarhan potash deposit in China. Their origins cannot therefore be identified simply based on the presence or absence of certain evaporite minerals. Concentrations of Mg2+, Ca2 + and SO24 − in seawater have varied dramatically over geologic time, resulting in alternating KCl-type and MgSO4-type evaporite deposits (Hardie, 1996). For example, Cretaceous and Paleocene seawaters were relatively rich in Ca2+ but depleted in Mg2 + and SO24 − in comparison to modern seawater (Fig. 4; Horita et al., 2002; Timofeeff et al., 2006). All potash deposits from these two geologic eras are of the KCl-type (Timofeeff et al., 2006). The Mengye potash deposit will therefore also belong to the KCl-type, with depleted MgSO4. The deposit contains mineral assemblages (predominantly halite and sylvite) consistent with KCl-type evaporite (Table 2), and similar to most global marine potash deposits of similar age. However, this does not suggest a marine setting for the Mengye deposit because similar deposits have formed in continental settings (e.g. the Chaerhan potash deposit in China). Predominant halite and sylvite can present either as pure primary crystals (Fig. 2b, c and d) or mixed with mud and gravels. Pure crystals can also exhibit mud inclusions (Fig. 2b, c, d and f), indicating a high degree of super-saturation vis-à-vis halite and sylvite in soft mud in shallow environments (Gornitz and Schreiber, 1981; Warren, 1999; Rahimpour-Bonab and Kalantarzadeh, 2005). Some red sylvite appears deposited through already-deposited halite (Fig. 2b). Primary sylvite and halite are granular and blocky and gray white, blue–gray, gray and orange–red in color. occurs.

M. Li et al. / Ore Geology Reviews 69 (2015) 174–186

Secondary fine sylvite and halite are mixed with mudstone conglomerates and are a mottled gray–green and purple–red in color, and are largely interconnected in a mesh-like formation (Qu et al.,

(b)

1998). This could be the result of the movements and deformations of salt bodies. Secondary salts reveal continental sedimentary facies.

0.05

Mengye halite after Xu (2008)

0.045

Basal halite after Siemann (2003) Mengye halite this study

0.04

Laotian halite this study

0.035

wt% Br in halite

179

0.03 0.025 0.02

recent seawater

synthetic seawater

0.015 0.01 0.005 0 0

0.5

1

1.5

2

2.5

3

3.5

4

Mole(Mg+SO4 /Ca+K)

Ion concentration in Mengye halite (%)

(c)

Ca%

3

Mg%

SO4%

K

R2 K= 0.09 2

R

Ca=

-0.144

R2 Mg = -0.292 R2 SO4 = -0.067

2.5 2 1.5 1 0.5 0 0

50

100

150 200 250 300 Br in Mengye halite (ppm)

350

400

450

Fig. 5. Factors on Br concentrations. (a), Molality of major ions (after Siemann (2003)); (b) and (c), relationship between Br in halite and (Mg + SO4)/(Ca + K); (d) variations of Br contents in basal marine halite in the past 500 Ma and potash deposit types (after Siemann (2003)). The Br data of Yunnan (Mengye potash deposit) in China was dated in the Paleocene, but it still disputed for the Paleocene or the Cretaceous.

180

M. Li et al. / Ore Geology Reviews 69 (2015) 174–186

Fig. 5 (continued).

Some trace/tiny salt secondary minerals have also been found in the area (Table 2). It is better to define an evaporite unit by the appearance in any proportion of a critical new mineral, rather than by its dominant minerals (Holser, 1979). Langbeinite (2MgSO4·K2SO4), for example, is stable only at temperatures greater than 40–50 °C (Stewart, 1963), implying that the minimum temperature experienced by potash-bearing halite in the Mengye deposit was above at least 40–50 °C. It also suggests that such minerals were generated during episodes of thermal metamorphism. The Sr- and K-bearing mineral kalistrontite (SrK2(SO4)2) is a rare evaporite mineral that has previously been reported from Permian deposits (Voronova, 1962), from a recent pan in Namibia (Mees, 1999), from a Pliocene Sedom Lagoon in the Dead Sea Rift Valley, Israel (García-Veigas et al., 2009), and from the Miocene Emet Basin in Turkey (García-Veigas et al., 2011). It forms from remnant interstitial brines with a high potassium and strontium content. The presence of secondary K-bearing minerals (kalistrontite, langeinite, rinneite and carnallite) indicates high K content in the interstitial brines. Rinneite (K2NaFeCl6) content is minuscule, although it has been found in many potash deposits such as in Eskdale, Cumbria, England; Wolkramshausen, Thuringia, Germany; Hildesheim, Lower Saxony, Germany; and Zechstein, Germany (Borchert, 1969; Smith and Crosby, 1979; Kampf et al., 2013). Previous studies have asserted that rinneite formed during metasomatic replacement processes by reaction with MgCl2-rich, bituminous solutions which originally had a higher iron and fluorine content than the chloride salt precipitates in their early stages, or as a secondary product of oxidation (Qu, 1997), or as a result of the evaporation of geothermal (hydrothermal) brines enriched in K, Na, Mn, and Cl (Kampf et al., 2013). Carnallite (KMgCl3·6H2O) can be formed both in non-marine and marine settings or be related to hydrothermal fluids (Qu, 1997). In the Mengye potash deposit, minor carnallite deposits are red, orange and white in color. Primary carnallite is enwrapped in halite or in the intercrystalline spaces as droplets and/or irregular grains. Secondary carnallite occurs in sylvite-bearing muddy gravels or is formed from interstitial brines; most present as fine veins or crumby masses (Qu et al., 1998). Celestite (SrSO4) occurs in various environments ranging from sedimentary (evaporitic and non-evaporitic) to non-sedimentary (e.g. hydrothermal) conditions (De Brodtkorb, 1989; Abidi et al., 2012). Much of the strontium which is incorporated into celestites probably originates from the dissolutions of strontium-rich carbonates (Baker and Bloomer, 1987) and hydrothermal fluids. Needle-shaped celestite was

present irregularly in the cements in the sylvite layers (Xu, 2008). Although celestite is a common mineral in ancient marine settings, it is also present in continental sediments of hydrothermal strontium origin, such as the Daqingsha sedimentary-evaporative deposit in the QB. Koenenite (Mg5[Na6Al2(Cl,Br)6·(OH)11·nH2O]) is only found in sylvitebearing muddy gravels as an unstable secondary mineral; it occurs as veins and irregular masses (Xu, 2008). Because of their continental host, these trace/tiny minerals indicate a continental origin. Boron minerals are commonly found in potash deposits (e.g., Grice et al., 2005; Grew et al., 2011; Grice and Rowe, 2014), formed in the latter stages of an evaporation sequence. By far the greatest diversity (80 species of borates) is found in evaporites, occurring in both nonmarine (e.g., Helvaci and Ortí, 2004; Smith and Medrano, 2002) and marine (e.g., Braitsch, 1971; Grice et al., 2005) environments. Minuscule traces of α-boracite (Mg3B7O13Cl) and parahilgardite (Ca2B5O9Cl H2O) in the Mengye deposit are found only in the blue–gray sylvite (Xu, 2008). Their presences are indicative of a B-rich brine supply subsequent to volcanic activities in the Mengye area (Qu et al., 1998). Typical continental minerals containing Na2CO3 are not found associated with these evaporites. Therefore, these minerals are not indicative of either marine or non-marine origins. 4. Geochemistry 4.1. Variation of Br, I, Br/Cl and I/Cl in halite Bromide (Br) and iodine (I) ions are highly soluble in natural water. They exist naturally in several oxidation states, ranging from Br− (or I−) − to BrO− 3 (or IO3 ). None of them participate in significant ion exchange reactions at low temperatures, nor are they adsorbed onto mineral surfaces, and they only form minerals during extreme evaporation conditions, when halite starts to precipitate (Fontes and Matray, 1993; Herczeg et al., 2001; Cartwright et al., 2006). Theoretically, the Br concentration in halite depends on the concentration of Br in the solution, degree and rate of evaporation, and other ions in the solution (Fig. 5a and b; Siemann, 2003). As the evaporation rate increases, the concentration of incompatible Br in the solution increases and more Br enters b 1). Our the crystalline structure as a substitute for Cl (Dsolid/solution Br study concurs with the work of Siemann (2003). For example, Fig. 6 shows that the concentration of Br correlates positively with the Br/Cl

M. Li et al. / Ore Geology Reviews 69 (2015) 174–186

(a)

181

(b)0.16 2400

0.14

2200

0.12

2000 1800

0.1

1600

Br /ppm

I (ppm)

M engye,this study

1400

Laos,this study

1200

M odern seawater

1000 A-1

Laos (Li,2008)

600

Mengye 0.06

M engye (Xu,2008)

800

0.08 Laos Modern seawater

0.04

Laos (Li et al., 2010)

400

0.02

Lao (Tan et al.,2010)

200 0

0 0

0.5

1 1.5 M olar ratio 10-3 Br/Cl

2

0

2.5

0.5

1

1.5

2

2.5

Molar ratio 10 -6 I/Cl

( a-1 )

(c)

0.68

800 0.45 0.31

30

600

400 Carnallite stage Sylvite stage 200

Laos

Mengye

25 -6

Halite stage

molar Br(10 )

Br /ppm

Final stage

0.1

20 15 10 5

0

0 0

(d)

0.1

0.2

0.3 0.4 -3 M olar ratio 10 Br/Cl

0.5

0.6

0.7

0

0.1

0.2 0.3 Total carbon (%)

0.4

0.5

0.6

3

-9

molar I (10 )

2.5 2 1.5 1 0.5 0 0

0.1

0.2

0.3 Total carbon (%)

0.4

0.5

0.6

Fig. 6. Values of Br, I and total carbon (TC), Br/Cl, I/Cl, and their relationships of potash deposits in China and in Laos. (a), Distribution of Br and Br/Cl; (b), relationship between I and I/Cl; (c), relationship between Br and total carbon; and (d), relationship between I and total carbon.

ratio, suggesting that as evaporation increases, more Br is incorporated into the halite crystal. Variations within the solution's chemistry can also dramatically change the partition behavior of Br between halite and halitesaturated brine. It is known that the SO24 − concentration exerts the 2+ and Mg2+ (Fig. 5 a). It greatest influence, followed by K+, HCO− 3 , Ca has been shown that the Br contents of marine basal halite correlate with the molar ratio (Mg + SO4)/(K + Ca) of seawater (Fig. 5b; Siemann, 2003). However, such a relation is not found in the Mengye halite (Fig. 5b). The correlation coefficients between Br and K are low (R2K = 0.09), and those of Ca, Mg and SO24 − are negative (Fig. 5c). This could be explained by the following possibilities: a) the Mengye ha− + 2+ lite in this study is not basal halite; b) changes in SO2− 4 , K , HCO3 , Ca and Mg2+ ions do not cause significant change in the quantity of Br in halite, this agrees with Holland et al. (1996); Br concentrations and Br/Cl ratios for Mengye halite are mainly influenced by the degree of − + 2+ and Mg2+ concentrations evaporation; and c) SO2− 4 , K , HCO3 , Ca were low at the Mengye halite precipitation stage. All basal halite Br contents in Fig. 5d as reported by Siemann (2003) are marine halite, including that from the Yunnan Basin (YB; Mengye potash deposit) and the KB (including the SNB). In our study, Laotian lowermost halite Br content is very high (275 ppm, Table 3, Fig. 5d). This could be because: (a) one of the two halite crystals in our study (no. GM-2-8, Fig. 2g; Table 2) and that cited by Siemann (2003), are

not basal halites; and (b) the Laotian potash is not a marine deposit. The origins of KB (including SNB) and Yunnan basin (Mengye) potash deposits are still under significant debate whether they are marine, continental or mixed. This study holds to the established opinion that Br concentrations in halite depend on Br concentrations in solution. The linear relation between Br and Cl (Fig. 6a) suggests that Br and Br/Cl values are influenced principally by the degree of evaporation. With increasing evaporation, Br is more often a substitute for Cl during the late rather than the early stages of evaporation, when the Br/Cl ratio increases. Br/Cl molar ratios have also frequently been used by many studies to determine whether the origin of water is marine or non-marine (Swihart and Moore, 1986; Utha-Aroon, 1993; Vengosh et al., 1995; El Tabakh et al., 1999; Taberner et al., 2000; Rahimpour-Bonab and Kalantarzadeh, 2005; Kendrick et al., 2006; Tan et al., 2010). The typical Br/Cl × 10−3 molar ratio is ~0.11 mmol/mol in the halite stage during seawater evaporation, 0.31 mmol/mol in the sylvite stage, 0.45 mmol/mol in the carnallite stage and 0.68 mmol/mol in the final stage (Fig. 6a-1, Cheng et al., 2008). Salt rocks with a continental or mixed origin have Br/Cl ratios about 10 to 100 times lower than those of seawater, and rarely exceed 0.1 mmol/mol (Warren, 1989). In the Mengye potash deposit, Br/Cl molar ratios for halite and sylvite are from 0.06 to 0.38 mmol/mol, less than those of seawater, though most are N 0.1 mmol/mol (Table 3, Fig. 6a). Molar Br/Cl ratios in previous

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Table 3 Major ion concentrations, halogen, total carbon (TC) and molar ratio of Br/Cl and I/Cl in salt minerals from Mengye potash deposit in China and in Laos. Iodine content of seawater is after Herring and Liss (1974). Number

Ca (mg/g)

K (mg/g)

Mg (mg/g)

Na (mg/g)

SO4 (mg/g)

Cl (mg/g)

Br (ppm)

I (ppm)

Molar Br/Cl (10−3)

Molar I/Cl (10−6)

Total carbon (%)

MY-670-0 MY640W-1R MY640W-1S MY670-W5 MY670W-1 MY670W-7 MY-670-1 MY-670 W-5 MY-syl GM-6 GM-7 GM-8 GM-3 GM-9 GM-10 GM-2-2 GM-2-3 GM-2-4 GM-0-2 GM-0-3 GM-4 GM-11 GM-12 GM-2-5 GM-2-6 GM-2-7 GM-2-8 Modern seawater

1.87 2.05 11.56 0.13 1.49 0.14 9.88 3.52 3 146.4 0.97 2.3 224.5 0.03 1.8 0.3 1.27 1.47 0.16 0.66 3.44 1.54 0.17 0.13 0.68 0.22 0.4 400

35.59 130.83 1.9 3.26 14.84 26.1 40.38 2.49 61.96 0.93 61.43 0.99 0.65 86.98 1.46 0.85 0.74 15.22 0.72 116.91 1.64 6.07 0.72 16.7 88.78 63.98 10.37 380

0.25 0.16 0.06 0.17 0.21 0.09 4.47 4.06 0.81 0.18 0.07 – 1 50 0.89 – – 0.38 – 0.29 0.54 3.54 0.06 0.55 0.3 37.12 4.78 1350

277.62 193.76 230.54 310.75 288.72 296.5 117.05 282.36 225.93 4.45 140 206.82 44.89 0.3 163.48 181.41 176.9 142.59 196.7 103.12 302.92 292.3 194.34 160.2 114.66 28.92 180.26 10500

3.99 4.54 30.02 – 4.3 – 17.84 7.96 6.79 860.3 4.2 10.07 761.81 – 4.63 1.19 5.41 5.48 0.66 2.98 8.24 3.6 0.59 0.35 2.72 0.86 1.46 2967

676.42 644.84 652.77 700.2 717.29 733.05 344.52 682.55 645.55 18.5 688.23 884.97 84.87 540.92 742.1 730.99 720.4 655.71 767.9 659.71 727.29 713.27 762.09 675.75 645.85 461.16 738.29 19.35

332 555.45 120.1 107.02 200.63 312.29 203.33 89.08 304.29 9.18 529.7 134.67 35.15 674.44 116.29 114.89 98.44 115.84 129.78 402.28 143.01 172.92 108.34 154.95 786.67 2129.5 275.19 67

0.1 – 0.07 0.27 0.23 0.22 – 0.22 0.16 0.14 0.21 0.23 0.18 0.09 0.13 0.29 0.2 0.14 0.32 0.22 0.14 0.07 0.07 0.2 0.23 0.24 0.3 0.06

0.04 – 0.03 0.11 0.09 0.08 – 0.09 0.07 2.12 0.09 0.07 0.59 0.05 0.05 0.11 0.08 0.06 0.12 0.09 0.05 0.03 0.03 0.08 0.1 0.15 0.11 1.1

0.22 0.38 0.08 0.07 0.12 0.19 0.26 0.06 0.21 0.22 0.34 0.07 0.18 0.55 0.07 0.07 0.06 0.08 0.07 0.27 0.09 0.11 0.06 0.1 0.54 2.05 0.17 1.54

0.02 0.03 0.14 0.01 0.01 0.28 0.17 0.25 0.09 0.03 0.13 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.01 0.01 0.08 0.03 0.02 0.03 0.03 0.01 0.01

Bold means values of halite and sylvite.

studies average ~0.11 mmol/mol in halite, and 0.73 mmol/mol in sylvite (Xu, 2008), suggesting a marine or mixed origin for the Mengye potash deposit. Half the Laotian Br/Cl ratios in this study are b0.1 mmol/mol, and only one exceeds that of seawater (Table 3, Fig. 6a), suggesting that more marine water was present in Laos than in China. Previous studies show that iodine concentrations are closely associated with an abundance of organic matter (Martin et al., 1993; Gilfedder et al., 2011). Iodine can be easily oxidized to HOI at the current PO2 level, being weakly bound with organic matter and easily released during the (micro) biological decomposition of organic matter (e.g. Gilfedder et al., 2010, 2011). I values and I/Cl molar ratios in the Mengye potash deposit are from 0.07 to 0.27 ppm, and from 0.03 to 0.11 μmol/mol, respectively (Table 3). These values are lower than those found in modern seawater, oil-field brines, hydrothermal brines and formation waters (Worden, 1996; Richard and Gaona, 2011; Wu et al., 2012). There is no relation between iodine concentrations and Cl (Fig. 6b), or between TC and I or Br concentrations (Fig. 6c and d). This could be because most of the iodine remain in residual brines and did not enter the structure of crystals because the total organic carbon content is low (ranging from 0.01 to 0.5%; Table 3), or because the brine has a continental origin with low iodine content. 4.2. Isotopic compositions of sulfur, boron and strontium The isotopic compositions of sulfur, boron and strontium can be more robust indicators of the origins of evaporites (marine vs. continental origins). Waters from continental sources in general have lower δ34S values than those from contemporary seawater. The δ34S values of freshwater usually range from −5 to 5‰ (Nielsen, 1972), while recent seawater values are globally uniform at 20‰ ± 0.5‰ (Thode et al., 1961; Lu et al., 2001). Sulfate minerals (such as gypsum and anhydrite) have stable δ34S values and are typically resistant to diagenetic alteration (Pierre, 1985; Lu et al., 2001; Bottrell and Newton, 2006; Alonso-Azcárate et al., 2006). Factors influencing the δ34S values of evaporative sulfates include contributions from marine and nonmarine sources, reservoirs and redox (Lu et al., 2001; Bottrell and

Newton, 2006). For example, bacterial-facilitated sulfate reduction is preferentially enriched in lighter 32S isotopes, and, therefore, the residual sulfate minerals (such as gypsum) enrich the heavy 34Sisotopes (Pierre, 1985; Lu et al., 2001). In contrast, reservoir effects can decrease the δ34S values of the subsequently-precipitated gypsum. If sulfate evaporites are precipitated in a closed system, δ34S values, as expected, display a gradual decrease upward in the vertical stratigraphic sequence. Average anhydrite δ34S values decrease from lower to upper strata both in Laos and in the LSB (Fig. 7a). This suggests that the reservoir effect played an active role in influencing the δ34S values of gypsum and anhydrite in the two basins. This provides possible evidence that the Mengye deposit brines came from Laos. The vertical sequence of δ34S values in a section from the Mengye potash deposit did indeed decrease from the lower to upper strata (Fig. 7a-1; Xu, 2008), suggesting a reservoir effect on the δ34S values. All δ34S values for gypsum and anhydrite from the LSB range from 3.85 to 15.33‰, lower than for Paleocene and Cretaceous seawater (21–22‰, 18‰), suggesting a continental origin. Those in the KB (including the SNB) were from 6.4 to 17‰, also lower than those of Cretaceous seawater (Fig. 7b), but nonetheless near to 18‰, suggesting a continental origin with possible residual seawater. Natural boron, a highly soluble element, has two stable isotopes (l0B and l1B) with approximate abundances of 20% and 80%, respectively. Boron-isotope fractionation is dependent upon the distribution of boron species, temperature and pH (Spivack and Edmond, 1987; Vengosh et al., 1992; Barth, 1993; Palmer and Helvaci, 1997), but pH is the key factor (Pagani et al., 2005; Paris et al., 2010; Foster et al., 2010). Dissolved boron exists mainly in the form of B(OH)3 and B(OH)4, which predominate as 11B(OH)3 at low pH values and as 10B(OH)4 at high pH values. Because pH values increase with increased evaporation, the δ11B values of minerals precipitated in the early stages of evaporation should be higher than those precipitated in the later stages. δ11B values may vary for different minerals, therefore. In accordance with the precipitation sequences of evaporites, the sequence of δ11B values is brine water N carbonate N gypsum N borate N halite N sylvite. Generally, in halite and sylvite boron is present as an inclusion (Xiao et al., 1992). Their δ11B values are almost exactly representative of boron isotope values for

M. Li et al. / Ore Geology Reviews 69 (2015) 174–186

183

( a-1 )

(a)

Anhydrite in Lower halite layer, Laos

Anhydrite in Middle halite layer, Laos

Anhydirte in Upper halite layer,Laos

Anhydirte in halite layer,Mengye, China

600

Average of every unit

500

( a-1 ) Mengye, China

Depth/m

400

300

Upper, Laos Middle, Laos

200

100 Lower, Laos

0 8 8

10

12

14

δ34 S of anhydrite (‰)

16

18

10

12

14

16

18

34

δ S of anhydrit of a section from Xu(2008)

Fig. 7. δ34S values from published papers. (a) δ34S values of Laos after El Tabakh et al. (1999); (a-1) δ34S values of a section of Mengye after Xu (2008); (b) δ34S curve of seawater from Timofeeff et al. (2006), and δ34S values from evaporites, sulfides and water.

contemporary paleo-brines when halite and sylvite are precipitated. They should therefore be higher than those found in carbonates and borate minerals. However, the δ11B values of halite and sylvite in this study are lower than those of borate minerals (Fig. 8). This could be because the paleo-brines have been dissolved when halite and sylvite were precipitated. Most of the halite and sylvite δ11B values in the Mengye potash deposit (−1.54 to 19.1‰, Zhang et al., 2011) are lower than those for the halite and sylvite in Laos (19.9–31.1‰) (Fig. 8). This implies that the paleobrines in the Mengye deposit were from Laos and dissolved during the flow process. The δ11B values of borates, and of halite and sylvite, are lower than those of the post-Cretaceous seawater (see the δ11B curve in Fig. 8), but some δ11B values of non-marine waters, such as hydrothermal fluid, saline lake water and river water, are higher than those of seawater. In other words, the δ11B values of borates and chloride minerals being lower than those of contemporary seawater mean that they are not indicative of continental origins. However, a comparison of δ11B

values between known marine minerals and unknown minerals is a useful way to distinguish between marine and continental origins. The δ11B values from Laos are lower than those of post-Cretaceous seawater, but are approximate to those of marine borates (Fig. 8, Swihart and Moore, 1986; Xiao et al., 1992; Palmer and Helvaci, 1997), which also suggests a marine origin. The δ11B values of the Mengye potash deposit are generally lower than those of the Laotian halite and sylvite (though some do approximate), suggesting a continental origin, with a possible remnant seawater trace. Strontium is a divalent alkaline earth element. Its isotope composition is almost homogeneous with modern seawater, with 87Sr/86Sr values of 0.709175 to 0.709235 (Hess et al., 1986; McArthur, 1994; Krabbenhöft et al., 2010). Because the Sr composition/isotope ratios for the weathering and erosion of rocks are high (Rahaman et al., 2011), the 87Sr/86Sr ratios for river water evince a large range, from 0.7074 to 0.803 (Fig. 9). In a continental setting, the 87Sr/86Sr ratios for continental waters are almost always higher than those for marine

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Fig. 8. δ11B values from published papers.

waters (Fig. 9; Denison et al., 1998). The 87Sr/86Sr values of Paleocene halite in the QB (a continental basin), for example, are higher than those of Paleocene seawater (Fig. 9). Thus, the 87Sr/86Sr values of authigenic minerals are highly variable, and certainly higher than those of seawater when minerals are precipitated from a solution. In the LSB, the 87Sr/86Sr values for celestite and anhydrite range from 0.71044 to 0.71243 and from 0.70777 to 0.71198, respectively (Fig. 9). The majority are higher than Paleocene (0.70772–0.70783, Hess et al., 1986) and Cretaceous seawater values (0.7072–0.70805, Prokoph et al., 2008) (Fig. 9), suggesting that the LSB has a continental origin. 5. Conclusions The Mengye potash deposit presents continental lithological features, even though the LSB experienced marine environments

during its evolution. The basin's evaporative minerals are dominated by halite and sylvite. The amounts of anhydrite, chlorocalcite, langbeinite, glaserite, tachyhydrite and glauberite are small. All these evaporites are formed both in marine and continental environments. The values of I and the I/Cl molar ratios of halite and sylvite range from 0.07 to 0.27 ppm, and from 0.03 to 0.11 × 10− 6, respectively, dependent upon organic substances, and so cannot be used to distinguish between materials of marine and continental origin. Br and molar Br/Cl ratio values range from 89.08 to 555.45 ppm and from 0.06 × 10− 3 to 0.38 × 10− 3, respectively, suggesting that the Mengye deposit is of mixed origin. Previously-published δ34S and 87 Sr/86 Sr evaporite values indicate a continental origin for the basin, while the δ 11 Bvalues indicate a remnant seawater trace. Thus, the potash deposit is of continental origin, with a remnant seawater trace.

Fig. 9. 87Sr/86Sr values from published papers.

M. Li et al. / Ore Geology Reviews 69 (2015) 174–186

Hydrothermal fluid could also be the origin of the potash deposit. Possible reasons for this may be: (1) faults or fracture systems were well-developed in the area during the evolution of the basin, creating deep input paths allowing hydrothermal fluid access to the basin; (2) hydrothermal fluids are an important source of deposits for several metals in the LSB and its environs, including one of the world's largest deposit of Zn–Pb (Li et al., 2004b; Xue et al., 2006; Huang et al., 2011; Zhang et al., 2013b). This suggests that the activity of hydrothermal fluids was widespread in the basin and thus a possible origin for the Mengye potash deposit; (3) several of the abovementioned trace evaporite minerals were formed by hydrothermal fluids; and (4) there is evidence for many hot springs in the LSB (Qu et al., 1998), supporting the notion that hydrothermal fluids had access to evaporites throughout their deposition history. 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